The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
The loading of tools or tool blanks (hereinafter “tools”) into the collet of a grinding or milling machine is usually either a manual operation, or a robot controlled operation. More sophisticated grinding or milling machines employ robot loading, which offers advantages in terms of reduced operating personnel and increased safety, by operating personnel being kept away from the grinding or milling interface and the environment within which grinding and milling takes place. The use of robot loading also allows the machinery to be largely enclosed, with the grinding or milling machine and the robotic loader all able to be enclosed within a single cabinet. The need for access to within the cabinet can be infrequent and that provides the advantage that operating personnel can be separated from the grinding or milling interface and protected from the moving parts of the machinery and from the cutting and lubricating oils that are used in the grinding or milling process and fumes that are created.
To load a tool into a collet by robot, the robot arm is calibrated so the arm accurately positions the tool for insertion into the collet. Once calibrated, the robot will continually position the tool to the calibrated position, unless environmental changes affect the dimensions of the robot arm, such as for example that the arm expands or contracts due to changes in temperature or humidity. Environmental changes can also affect the dimensions of the collet or the grinding or milling machinery to which the collet is fixed. For example, many factories within which relevant grinding or milling machines operate, are not temperature controlled, so the temperature can fluctuate during the day. For example, it is common for the environment within which the machines operate to increase in temperature and/or humidity from morning to afternoon and such environmental changes can be sufficient to affect the accuracy with which a robot arm positions a tool for insertion into a collet. Even only small dimensional changes can be sufficient to affect the dimensions of the robot arm in a manner that becomes problematic as explained below. The change in accuracy will not necessarily affect arrangements where the clearance between the tool and the collet is large, but where there is very low clearance, such as between 0 to 20 microns, the change in accuracy can result in either failed loading such as jamming of the tool within the collet, or incorrect loading, the latter of which can result in inaccurately or wrongly machined parts.
For example, where an inaccuracy occurs in loading a tool into a collet so that the longitudinal centre line of a tool becomes misaligned with the centre line of the collet, the tool can engage against one side of the collet and can tend to bend. This is because the robot arm will force the tool into the collet at the position it has been calibrated to load the tool, regardless that the tool is not correctly aligned with the centreline of the collet for loading. The rigidity of the collet and the robot arm provide no potential for movement of the tool into an aligned position, so that the tool will be loaded at a slight angle within the collet, or bent as it is forced into the collet, or both. Because the misalignment can be so small, it will not be easily detectable, and often only becomes apparent once the tool has been machined. If the inaccuracy is not discovered for some time, many tools can be inaccurately machined resulting in significant wasted time and materials.
In more significant conditions of misalignment, the tool can be positioned to load completely offset from the opening of the collet, so that when the tool is advanced by the robot arm to insert the tool into the collet opening, the leading end of the tool engages the face of the collet about the collet opening, but does not enter the opening. If this happens, it is clear to the operator that recalibration is required.
It will be appreciated that once the robot arm is out of alignment, every tool which is loaded will be loaded inaccurately. Only the extent of the misalignment might change. Accordingly, it is the case that calibration of the arm needs to occur from time to time, particularly where significant temperature and/or humidity changes occur throughout the day, and that means down time for the machinery and increased personnel interaction with the machinery. In addition, there is no ability to check when recalibration is required. Thus, recalibration occurs as the machine operator decides or according to a predetermined schedule, or when it is clearly apparent that tools are not being loaded correctly or are being machined inaccurately.
Robot arms that are set up to load tools into collets for grinding or milling, tend to retract and extend in a plane which is approximately perpendicular to the axis of the collet. Such robot arms are usually also set up to extend and retract generally horizontally rather than at an angle or vertically. Such arms which operate horizontally tend to have a small movement or “compliance” in the vertical direction, but are very stiff in the horizontal direction and therefore lack movement or compliance in that direction. Thus, inaccuracy in the placement of the tool for loading into the collet tends to be accommodated in the vertical direction by the allowable movement or compliance in the robot arm, but there is no such accommodation in the horizontal direction. In such arms as discussed above, it follows that it is lack of compliance in the horizontal direction that results in misloading of a tool in the collet and subsequent inaccurate machining, or a complete failure to load because the tool is positioned to load offset from the opening of the collet.
In contrast, if a robot arm was set up to extend and retract generally vertically, then the available compliance would be in the horizontal direction and not in the vertical direction. In effect, the available compliance is generally in the direction perpendicular to the direction in which the robot arm extends and retracts and the need for compliance is in the direction in which the robot arm extends and retracts.
Applicant has therefore recognised that the development of a tool gripper which can be mounted to a robot arm and which provides for compliance in the direction in which the robot arm extends and retracts, would be desirable.